Generally, particulates (particulate matter) from a diesel engine in an automobile is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contains a trace of sulfate (misty sulfuric acid fraction). For reduction of such particulates, it has been carried out to incorporate a particulate filter in an exhaust pipe through which an exhaust gas flows.
Such kind of particulate filter has a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets have plugged outlets. Thus, only the exhaust gas passing through thin porous walls compartmentalizing the passages is discharged downstream.
The particulates in the exhaust gas, which are captured and accumulated on inner surfaces of the thin porous walls, require to be burned off for regeneration of the particulate filter before exhaust resistance increases due to clogging. However, exhaust gas from a normal engine in an automobile rarely has a chance to reach a temperature level at which the particulates ignite by themselves, so that employed is a catalyst-regenerative particulate filter integrally carrying an oxidation catalyst.
Specifically, such employment of the catalyst-regenerative particulate filter facilitates an oxidation reaction of the particulates captured to lower an ignition temperature, so that the particulates may be burned off even at an exhaust temperature level lower than ever before.
However, even in such employment of the catalyst-regenerative particulate filter, a captured amount may exceed a treatment amount of the particulates in an operation area having a lower exhaust temperature level. Continued operation at such lower exhaust temperature level may hinder sufficient regeneration of the particulate filter, possibly resulting in excessive accumulation of the captured particulates in the particulate filter.
Thus, it has been conceived that a flow-through oxidation catalyst is additionally arranged on an entry side of the particulate filter; with the accumulation of the particulates becoming increased, fuel is added to the exhaust gas upstream of the particular filter to regenerate the particulate filter.
Specifically, the addition of the fuel upstream of the particulate filter in post injection or the like generates HC gas which causes an oxidation reaction on the oxidation catalyst of the particulate filter. Heat of the reaction increases a bed temperature of the catalyst to burn off the particulates, thereby regenerating the particulate filter.
Specific measure for such fuel addition may be such that main injection of the fuel near a compression top dead center is followed by post injection at non-ignition timing after the top dead center to add the fuel into the exhaust gas.
Nowadays application of such particulate filter not only to an automobile but also to a crane, a shovel or other constructing machine has been reviewed. It has been conceived that an accumulation amount of the particulates is estimated on the basis of, for example, a cumulative amount of a difference between estimated generation and treatment amounts of particulates calculated from a difference of pressures before and after the particulate filter, an engine rotation frequency and a load, and regeneration of the particulate filter is automatically conducted when the estimated value exceeds a set value.
However, even if the automatic regeneration control of the particulate filter is conducted during a work, the regeneration of the particulate filter may be not always completed depending on an intermittent situation of the work; repetition of such situations may increase the accumulation amount of the particulates in the particulate filter.
Thus, it is required that, in case of a great amount of particulates being accumulated in the particulate filter, regeneration of the particulate filter can be arbitrarily conducted even not during the work depending on an operator's intention (automatic control with a proper flagging may be alternatively used). However, trial to regenerate the particulate filter in an idling state not during the work would result in vain since a sufficient oxidation reaction of HC gas on the oxidation catalyst were unexpectable because of too low exhaust temperature.
For example, in a case of an automobile, a technique has been suggested that a temperature of the exhaust gas is increased by increasing an idling rotation frequency during idling while an exhaust brake or an intake valve is closed. However, in a case of a constructing machine, parts such as the exhaust brake and the intake valve are not installed unlike the case of the automobile; to newly provide these parts only for regeneration of the particulate filter would result in substantial increase in cost.
Moreover, even if the exhaust brake and the intake valve are newly provided to conduct exhaust or intake throttling, substantial increase in temperature of the exhaust gas is unexpectable in the idling state, so that it takes much time to complete the regeneration of the particulate filter, unavoidably resulting in increase in cost due to increase in an added amount of the fuel.
In order to overcome these disadvantages, the inventors developed a technique of effectively regenerating at low cost a particulate filter arranged in an industrial engine such as that mounted on a constructing machine or the like for driving a hydraulic or other work unit using engine power to conduct various works (see, for example, Patent Literature 1).
Patent Literature 1 discloses that for regeneration of the particulate filter, forced load application to the work unit intentionally increases an engine load, and a post injection or other fuel addition is conducted with the exhaust temperature being increased by the increased load.